Electron impact ion sources for mass spectrometers; typically comprise an ionisation region into which a gaseous sample is introduced, means for introducing electrons into the ionisation region so that the electrons ionise the sample, and means for extracting ions from the ionisation region. There are two types of electron impact ion source, the enclosed ion source and the open ion source. The open ion source is commonly used for applications involving gaseous samples contained in volumes substantially greater than that of the open ion source, for example as shown in Beatty, Greer and Kay in Med. & Biol. Eng. & Computing, 1981, Vol 19, p770-774. The ionisation region of a typical prior open ion source is surrounded by a pumped enclosure into which the sample is introduced. The ionisation region is defined by a cage through which electrons and molecules of the gaseous sample are able to pass. A filament:, which is maintained at a negative potential relative to the cage, is mounted outside the ionisation region and the potential difference between the cage and the filament is typically 20-70 V. An electric current is driven through the filament, to heat it, by a filament power supply. Electrons, which are generated by thermionic emission from the filament, are accelerated by the potential difference between the cage and the filament. The electrons pass into the ionisation region and collide with the sample to generate ions from. the sample. Power control means are typically provided to monitor the electron current at the cage and control the filament current, thereby maintaining the electron current at a preselected value. The ions may be extracted by a weak electric field which penetrates through an extraction aperture formed in the cage. The electric field is generated by the application of a potential difference to electrodes mounted outside the ionisation region and adjacent to the extraction aperture. For an open ion source incorporated into a mass spectrometer, ions are extracted from the source before being transferred to a mass analyser.
The gaseous sample is able to pass freely through the cage of the open ion source. The pressure in the region surrounding the filament is thus equal to the pressure within the ionisation region and the filament is exposed to the sample under analysis. The sample may be highly corrosive and this may result in degradation and ultimate failure of the filament. The possibility of filament failure is taken into account in many open ion sources by the provision of an auxiliary filament mounted adjacent to the cage. The auxiliary filament is not used to generate electrons unless the other filament burns out. The exposure of the filaments to the sample limits the pressure at which open ion sources may operate to the range 10.sup.-4 to 10.sup.-12 torr.
The second type of electron impact ion source is the enclosed ion source, an example of which is described in European patent specification 311224. The ionisation region of such an enclosed ion source is substantially enclosed by a wall to form a chamber. The wall is comprised of a solid electrically conducting material. A heated filament is mounted outside the ionisation region as described above. A gaseous sample which is introduced into the chamber is excluded from the region surrounding the heated filament by the wall. Typically the potential difference between the wall and the heated filament is in the range 20-70 V so that electrons are accelerated into the chamber through an electron entrance aperture formed in the wall. The electrons interact with the sample to generate ions from the sample. A second aperture may be formed in the wall so that the electrons can pass from the chamber to an electron trap. Power control means are typically provided to monitor the electron current at the trap and control the filament current, thereby maintaining the electron current at a preselected value. The ions are extracted through an ion extraction aperture formed in the wall, in the same way as described for the open ion source. The wall defines a substantially equipotential region, resulting in the formation of substantially monoenergetic ions.
Since the ionisation region is substantially enclosed, the pressure within the ionisation region may be much higher than for an open ion source (typically 0.5 to 1 mtorr) while the pressure in the region of the heated filament can be maintained at a much lower value, for example 10 torr or less. The heated filament is also separated from the sample, avoiding contamination and corrosion off the filament and increasing its lifetime. Enclosed ion sources are more suitable than open ion sources for analysis of a limited quantity of gaseous sample.
The operation of an enclosed electron impact ion source may be affected by the undesirable effect of impurities, such as organic materials or oxides, which can build up on the inner surface of the chamber. This may be due to the exposure of the chamber to gaseous samples under analysis or from diffusion of contaminants within the material of the wall. Such impurities may form an insulating film which can become electrically charged, seriously reducing the source performance, for example by causing potential gradients in the ionisation region which increases the energy spread of the ions. Further, this effect is mass dependent. The problems associated with contamination of electron impact ion sources are discussed in U.S. Pat. No. 4,481,062 and a number of solutions to this problem have been suggested. For example, the construction of a source to facilitate cleaning is outlined in U.S. Pat. No. 4,481,062. C. Chen et al, in Vacuum, 1984, Vol 34, No. 5, p 581-584, suggest that baking a source at 250.degree. C. for 48 hours is beneficial in reducing outgassing, and that electropolishing certain source components reduces the adsorption on the surface of those components.